The general techniques for practicing the art are thoroughly described in U.S. Pat. No. 7,425,296 (identified above) and the related patent family. The above family of patents generally teaches a technology which is known as narrowband digital heat injection by absorption spectrum matching, or for short, Digital Heat Injection, or DHI. An important DHI concept that must be well understood is that of matching the wavelength of irradiation to a particular wavelength at which the target has an absorption coefficient that is the most desirable for the desired application result. Since each type of material has its own unique absorption spectrum caused by the atomic absorption characteristics of its molecular makeup, it is necessary to understand what the absorption spectrum curve looks like for any given target material which is to be treated by way of DHI. The locus of points representing the complete set of absorption coefficients for each wavelength of irradiation will comprise the complete absorption curve for that material. The complete spectral absorption curve is often also referred to as the spectral curve or by other shortcut names. As a practitioner reduces DHI technology, to actually practice for a given application, there is a wide range of things to consider, as it is much more completely described in the '296 patent family referenced above.
While the term narrowband appropriately applies to all DHI applications, some applications are much more critical than others. For example, in some applications, two or three hundred nano-meters of bandwidth may be narrow enough to match a particular area of a given product's absorption curve. While each and every different material or compound has its own characteristic absorption curve shapes, they are often slow changing shapes in part of the curve and sharp or abruptly changing shapes in other portions of the curve.
Since every different type of material has its own characteristic curve shapes it is difficult to generalize, but while some materials will have gently changing absorption curves, many other materials will have areas of their absorption curve some place between UV and long infra-red which have rapidly or abruptly changing shapes. These will be areas of the absorption curve which have a very steep slope such that a small change in the wavelength equates to a very large change in the absorption coefficient. For example, pizza dough, water, pepperoni, and cheese all have active and rapidly changing curves in the 900-1500 nano-mater range wherein there are points at which less than a 50 nano-meter change in wavelength will yield a 3× to 5× difference in absorption coefficient. There are other materials, such as polyethylterylphthalate (or PET) material from which beverage and food containers are blown, which have portions of their absorption curves which are extremely steep. Targeting the exact point on such a steep curve in order to take advantage of the exact absorption coefficient which is optimal for heating the material in a desired way, requires a laser device that can be manufactured economically to a very high level of wavelength precision that is very repeatable. Similarly, if one is trying to hit a narrow peak or dip in the absorption curve (typically plotted using absorption on the y-axis and wavelength on the x-axis), wavelength precision is also required. The penalty for wavelength variation away from the desired center wavelength in such a case means that the irradiation would miss the peak and actually hit the target with energy that will be at a substantially different absorption than planned. The result would require a large change in the amount of energy required to achieve the desired heating or energy deposition.
Another concept of digital heat injection involves choosing wavelengths for a desired result when multiple different material types are involved. For example, choosing materials which have at least one wavelength at which the two materials have desireously different absorptions. When one material is highly transmissive at a wavelength at which the other is highly absorptive, it is possible to shoot the energy through a first transmissive material with minimal heating while achieving substantial absorption in the second material with a desired level of heating. This concept can be extended for more than two materials but the level of wavelength precision can rise even further. Additives can also be used which induce a high absorption peak to enhance the useability of this concept but it may further require high levels of wavelength choice and precision to accomplish the desired systemic result.
An important and often critically fundamental concept behind DHI technology involves choosing the right wavelength to have the precisely desired amount of absorption in a target. As has already been taught in the '296 patent family indicated above, the practitioner of digital heat injection will often want to choose two, three, or more wavelengths because each of them has a desirable absorption co-efficient at its respective wavelength. By irradiating with chosen proportioning, this allows a skillful practitioner to specify the exact combination of penetration and absorption that might be ideal for a given application. While DHI technology may work with reduced wavelength precision, it has been found that a substantial improvement can be made in the practice of the technology by incorporating a much higher level of wavelength precision. It has also been discovered that, certain specialized types of semi-conductor hardware may be necessary to further optimize the implementation and hit the precisely desired wavelengths with extremely narrowband energy and to accomplish it economically. Since lasers and other narrowband irradiation sources used for many DHI applications must be of a type and design that they can be manufactured and implemented economically in order to achieve broad commercialization, it is important to choose such lasers, LEDs or other narrowband emitting devices and manufacturing processes carefully.
Although nearly any type of laser or narrowband irradiator can be used to practice digital heat injection technology if it can be manufactured at the correct output wavelength for an application, there are certain practicalities which dictate a preference for certain types of irradiators for the desired application. In general, semi-conductor lasers which are also known as diode lasers, tend to be more practical because they lend themselves to the lower cost high production manufacturing. They also offer the ability to manufacture them at a much wider range of specific wavelengths, greater compactness, survivability, electrical efficiency, ruggedness, and other virtues.
Typical diode or semi-conductor lasers however, have certain limitations and manufacturing challenges as well. One troubling issue is the normal process variations that occur during manufacturing can cause the final laser devices to have a wider range of output wavelengths than is desired. Many thousands of devices are made on a single manufacturing ‘wafer’ or substrate disc. It is not unusual for the wavelengths of devices that come from the same wafer to vary randomly by +/−10 nano-meters or more, even for a process that is well controlled. They may be distributed with a normal statistical distribution around the mean or they may be heavily skewed in either direction from the targeted/desired center wavelength. If it is desirable to hit a specific center wavelength very precisely, like +/−1 or 2 nano-meters, the only choice is normally to sort the devices individually and pick only the devices which are in the tight desired range. This can mean that perhaps 80% or more of a production lot would need to be thrown away. Of course, sometimes they can be used for another application that needs an adjacent wavelength, but this is not a reliable business plan for most situations. This sorting procedure could easily cause the production yield to be below 20% when all the other production causes for fall-out are included. This is a major problem for high-production, high-powered use of such devices. Producing vast numbers of devices that are at the specified wavelength is required for the best economics and for solid commercialization of various products which may desire to apply DHI technology.
The design of traditional diode lasers necessitates a number of manufacturing steps which make it more expensive to integrate into an application and more expensive and involved to automate. The first aspect is that most diode lasers are chemically fabricated in an MOC-VD wafer fabrication machine with a layering approach. The ultimate lasing direction of each device is typically parallel to the plane of the wafer. The thousands of devices that are yielded from a single wafer by either saw cutting or scoring and cleaving to dice them into individual devices. Sometimes instead of cutting them into individual devices they are left physically connected as a row of devices which is then known as a laser bar. The bar may contain N lasers but might typically be 20 or more different laser devices, each of which functions individually. They are still mechanically joined to their neighbors because they were never separated from them. Whether it's a ‘bar’ configuration or whether it's an individual laser diode device for conventional ‘edge emitting’ lasers, it is necessary to perform polishing and other processes to the edges or ends of each device, one of which will become the emitting facet. The vast majority of all diode lasers are manufactured as these ‘edge emitting type’ devices. In an improved the design, all this additional processing and care for the edges would beneficially be eliminated from the manufacturing process in order to eliminate production steps and costs.
With reference to FIG. 5, typical edge-emitting devices 10 are shown in a bar 12 disposed on substrates 14 and 16. Substrate 14 (and/or 16 in some applications) may be a cooling substrate or system. Also, line D shows the general direction of the beam as it is generated in the wafer—to be output ultimately at a facet 20. The emitting facet 20 (three examples of which are shown) is the surface which ultimately is the site of the most common cause for failure in laser diodes. The emitting facet 20 is fragile and critical to the life of a laser diode. Any nick, scratch, imperfection, contaminant and some other issues on that surface can lead to additional local or large scale heating which in turn leads to failure. This is known typically as ‘catastrophic facet failure’ and is the most common failure mode in semi-conductor lasers. Also, the facet is generally rectangular in shape so issues of control and output consistency arise relative to the fast and slow axes of the laser output.
With reference to FIGS. 6(a) and 6(b), another problem encountered during the manufacturing mounting of traditional edge emitting laser devices 10 is the following. To maximize the life and output of diode lasers, it is necessary to cool them adequately and evenly. Lasers that put out any substantial amount of power should properly be mounted to some sort of heat dissipating substrate, e.g. substrate 14, on at least one side of the laser diode. For best cooling and maximum device life, the surface of the facet 20 must be absolutely flush and parallel (as shown by device 10-2 of FIG. 6(b)) to the edge of the heat sinking, cooling substrate 14. If the laser diode is at any skewed angle relative to the edge of the substrate or is not nearly perfectly flush (FIG. 6(a)), bad things begin to happen from a cooling standpoint which leads to early failure. If any portion of the substrate 14 (for example) protrudes beyond the facet surface by a distance N then it creates a location where contaminants can reside (as shown by device 10-3 of FIG. 6(b)) and the protruding substrate becomes a reflector/absorber of stray rays which come out of the emitting facet. Both conditions can lead to substantial additional heating of the facet material nearest the substrate. Also, if the facet 20 protrudes beyond the plane (by a distance M) of the cooling substrate 14 as shown by device 10-1 of FIG. 6(b), it prevents the substrate from sinking heat out of the laser device which can also lead to uneven heating and overheating of the critical facet area of the laser diode. Similarly, any interface medium or coating which has been superimposed between the cooling substrate or cooling circuit board and the laser diode(s) can either not come all the way out to flush or could ooze out and cause an overhang material situation. This also, like the other conditions, can lead to or contribute to catastrophic facet failure. To eliminate these problems it would be very desirable to incorporate a laser diode which can be mounted quickly and cost effectively without concern for the issues just described.
Since many DHI applications utilize more than one laser diode in order to get enough radiated energy to the target, the mounting complications and number of diodes required can raise the cost of manufacturing DHI systems substantially. Hence, another limitation of the current technology is the limited power that can be produced from a single laser diode. If the laser diodes are driven harder or designed as larger packages in order to get more power output, it raises the power density that must pass through the output facet. As the power density rises, the heat that is inevitable must be dissipated more carefully. The compromise that is often taken de-rates the devices to keep the efficiency and longevity reasonable.
Carefully controlling the temperature of the laser devices or laser arrays is not only critical to the life of the devices but it is critical in other ways as well. As the temperature of laser diodes goes up, the radiant output goes down. Also, as the temperature changes, the wavelength of the radiant output of the laser diode device changes as well. For most traditional semi-conductor lasers the output changes by 0.3 nanometer per degree centigrade change in junction temperature. This is problematic because in a DHI system it is more expensive and may use more energy to precisely control the temperature of the devices.
The list of substantial issues, as detailed above, are challenges that a practitioner of digital heat injection technology will encounter when trying to commercialize a system which fundamentally is built around conventional, edge emitting laser diodes and some other narrowband devices economically which gave rise to the novel thinking represented by the present invention.